<div style="height:60px; width: 250px; margin: 0 auto; text-align: center" data-bind="dxSparkline: {           
    dataSource: observableChartDataSource,
    argumentField: 'time',
    valueField: 'velocity',
    type: 'spline'
}"></div>

<div style="margin:5px; text-align:center">
    <div style="height: 40px" data-bind="dxButton: {
        text: 'Start',
        onClick: start,
        disabled: isStarted()
    }"></div>

    <div style="height: 40px" data-bind="dxButton: {
        text: 'Stop',
        onClick: stop,
        disabled: !isStarted()
    }"></div>
</div>